Parkinson's disease (PD) is the second most common progressive neurodegenerative disorder. The clinical manifestations of PD include resting tremors, rigidity, bradykinesia and postural instability with cognitive and emotional disorders. The primary characteristic pathology of PD is the loss of dopaminergic neurons in the substantia nigra pars compacta and the presence of intracytoplasmic inclusions known as Lewy bodies. The etiology and the pathogenesis of PD are not completely known but recent evidence suggest that environmental and genetic factors might account for the progression of this disease (Schapira, Lancet Neurol., 7: 97-109 (2008)). PD is considered to be a sporadic disease but recently several susceptive genes have been identified to be associated with familial forms of PD. Three missense point mutations (A53T, A30P and E46K) and genomic duplication or triplication of the α-synuclein gene have been reported as a cause of familial PD (Lee and Trojanowski, Neuron, 52: 33-38 (2006)).
α-synuclein is a protein that is predominately expressed in neurons, especially at synaptic terminals. The functions of α-synuclein are not well defined but it is reported to have potential roles in synaptic function and neural plasticity. A knockout animal model, used to examine the role of α-synuclein, was observed to be viable and have normal synaptic structure and brain morphology. These studies indicate that α-synuclein may modulate dopamine release, synthesis or storage. It may also act as a regulator of synaptic plasticity (Lotharius and Brundin, Hum. Mol. Genet., 11: 2395-407 (2002)).
In addition to PD, α-synuclein has also been identified as a major component of Lewy bodies and Lewy neurites in dementia with Lewy bodies (DLB), Alzheimer's disease (AD), pure autonomic failure (PAF), multiple system atrophy (MSA), and other neurodegenerative disorders (Lippa et al., Am. J. Pathol., 153: 1365-70 (1998), Marti et al, Mov. Dis., 18:S21-S27 (2003), Norris et al., Curr. Top. Dev. Biol., 60: 17-54 (2004)). The pathological α-synuclein exists as insoluble, filamentous aggregates containing abnormally nitrated, phosphorylated, and ubiquitinated residues in Lewy bodies and Lewy neurites. This new discovery has established α-synucleinopathy as an essential pathogenic feature of neurodegenerative diseases. It has been suggested that α-synuclein proteins have high propensity to adopt various conformations, with a strong tendency to self-aggregate into oligomers, which further aggregate into fibrils that are deposited as Lewy bodies and other similar pathologies. The mutant forms of α-synuclein are more inclined to aggregate as demonstrated in vitro and in animal models (Conway et al, Nat. Med., 4: 1318-1320 (1998); Giasson et al, Neuron, 34: 521-233 (2002)).
Additionally, it is thought that α-synuclein protein levels increase with age in the human substantia nigra (Li et al., J. Neurosci., 24: 9400-9409 (2004)). The connection between α-synuclein and neurodegenerative phenotypes in human patients and animal models strongly highlight the significance of the expression levels and the abnormal aggregation of this protein in the pathogenesis of PD. A53T α-synuclein transgenic mice under the control of the mouse prion-related protein promoter shows a marked and ultimately fatal motor paralysis with advancing age. Their motor neurons exhibit axonal degeneration near fibrillary α-synuclein inclusions, which reminisce part of the structure of the Lewy bodies (Giasson et al., Neuron, 34: 521-233 (2002)). A wealth of evidence suggest that the aggregated insoluble oligomer (protofibril) of α-synuclein plays an important role in the pathogenesis of PD. The assembly of the misfolded proteins in the form of oligomers leads to synaptic dysfunction, neuronal apoptosis and brain damage and underlies the pathogenesis of PD. Lansbury and co-worker demonstrated that α-synuclein protofibril forms elliptical or circular amyloid pores that can puncture the cell membrane which results in the release of the cell contents and cell death (Lashuel et al., Nature, 418: 291 (2002)).
Currently, there are no specific treatments that either halts or reverses the progression of PD. Commercially available drugs only relieve the symptoms of the disease to improve quality of life in PD patients. Since there are a wide range of symptoms and complications in PD patients, the choice of medication varies considerably between individuals. The most frequently prescribed medication for the treatment of PD is the use of drugs that boost the production of dopamine in the brain. Levodopa, which is modified by the brain enzyme to produce dopamine, is the most common medication for PD. Over the years, a number of drugs have been developed including dopamine agonists. However, the effectiveness of the drugs lessens after a period of treatment. Furthermore, some patients report side effects such as gastrointestinal ailments and psychological and cognitive problems (e.g. confusion, hallucinations, psychosis, etc).
Recent studies on the brains of PD patients have identified a loss of mitochondrial complex I function and generation of oxidative stress (Schapira, Lancet Neurol., 7: 97-109 (2008)), which are thought to play a part in the progression of selective nigral dopaminergic degeneration in PD. The biochemical defects in PD patients resemble the findings in the animal model of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). MPP+ (1-methyl-4-phenylpyridinium), an active metabolite of MPTP, is a neurotoxin and has widely been used in in vitro experiments to cause parkinsonism. It is taken up through the dopamine transporter and accumulates in dopamine neurons. The absorbed MPP+ concentrates in the mitochondria and inhibits the activity of complex I of the electron transport chain resulting in the reduction of ATP generation and the production of reactive oxygen species (ROS), and consequently leads to selective dopaminergic neuronal death resulting in PD-like symptoms.
Several studies have reported to achieve parkinsonism in the human SH-SY5Y cell line using MPP+, thus generating a PD model for primary assessment of new therapeutic compounds (e.g., Kim et al., Br. J. Nutr., 104: 8-16 (2010); Sun et al., Eur. J. Pharmacol., 660: 283-90 (2011)). A simple cell-based PD model is more applicable for a preliminary screen of potential therapeutic candidates before clinical trials in mammals for the prevention and treatment of PD.
In addition to MPTP/MPP+, 6-hydroxydopamine (6-OHDA) is another chemical that has been broadly used to induce parkinsonism in experimental animals (Betarbet et al., Bioessays, 24: 308-18 (2002); Lane and Dunnett, Psychopharmacology (Berl), 199: 303-12 (2008)). It enters the neurons via the dopamine and noradrenaline reuptake transporters and therefore often used in conjunction with a selective noradrenaline reuptake inhibitor (such as desipramine) to selectively kill dopaminergic neurons. It is considered to be an endogenous toxin since it was found in the urine of a PD patient (Andrew et al., Neurochem. Res., 18: 1175-7 (1993)) and oxidation of dopamine can lead to the generation of 6-OHDA in vitro (Napolitano et al., Chem. Res. Toxicol., 12: 1090-1097 (1999)). A wealth of evidence indicates that 6-OHDA generates reactive oxygen species and reduces the activities of glutathione and superoxide dismutase. Following intracerebral injection of 6-OHDA, striatal neurons begin to degenerate in 24 hr and striatal dopamine is depleted in 2-3 days.
N-methyl-D-aspartate (NMDA) receptors are ligand-gated ion channels located primarily within the central nervous system (CNS). They belong to the family of ionotropic glutamate receptors and are involved in neuronal communication and play important roles in synaptic plasticity and mechanisms that underlie learning and memory. Under normal conditions, NMDA receptors engage in synaptic transmission via the neurotransmitter glutamate, which regulates and refines synaptic growth and plasticity. However, when there are abnormally high levels of glutamate (i.e. under pathological conditions), NMDA receptors become over-activated, resulting in an excess of Ca2+ influx into neuronal cells, which in turn causes excitotoxicity and the activation of several signaling pathways that trigger neuronal apoptosis. Glutamate-induced apoptosis in brain tissue also accompanies oxidative stress resulting in loss of ATP, loss of mitochondrial membrane potential, and the release of reactive oxygen species and reactive nitrogen species (e.g. H2O2, NO, OONO−, O2−) causing associated cell damage and death. Decreased nerve cell function and neuronal cell death eventually occur.
Over-activation of the NMDA receptors is implicated in neurodegenerative diseases and other neuro-related conditions as it causes neuronal loss and cognitive impairment, and also plays a part in the final common pathway leading to neuronal injury in a variety of neurodegenerative disorders such as amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease and Huntington's disease, as well as conditions such as stroke. Recent findings have implicated NMDA receptors in many other neurological disorders, such as multiple sclerosis, cerebral palsy (periventricular leukomalacia), and spinal cord injury, as well as in chronic and severe mood disorders (Pogacić and Herrling, Neurodegener. Dis., 6: 37-86 (2009)).
There remains a need for compounds and compositions that can protect neuronal cells from MPP+/6-OHDA and NMDA induced cell death, and/or inhibit the aggregation of α-synuclein, or which are otherwise useful for the treatment of neurodegenerative and neuropathological diseases such as Parkinson's disease or other diseases associated with α-synuclein aggregation.